Exhaust gas recirculation (“EGR”) is used in diesel fuelled compression ignition engines to help reduce nitrogen oxide (NOx) emissions. EGR can reduce the concentration of oxygen in an intake charge entering the combustion environment to a level below the atmospheric concentration of oxygen. In EGR, a quantity of exhaust gas from one combustion cycle is retained in or routed back to the combustion chamber in a subsequent combustion cycle. The exhaust gas dilutes the oxygen in the intake charge.
An engine utilizing EGR typically starts with an intake charge that has an atmospheric oxygen concentration as it is drawn almost completely from the air. Oxygen is consumed during combustion of fuel. Exhaust gases from the combustion are depleted in oxygen. Consequently, where exhaust gases resulting from such combustion are mixed with an air intake charge, the concentration of oxygen within that charge is reduced.
It is well known that the use of EGR in diesel-fuelled compression-ignition engines can cause the engines to produce other pollutants. Combustion efficiency is the efficiency with which energy of a combustion event is converted into mechanical energy. As the oxygen concentration within the combustion environment falls, higher injection rates tend to be necessary to maintain combustion efficiency. The only practical ways to increase injection rates tend to result in increased emissions of particulates. EGR therefore has limited utility in reducing NOx emissions in current diesel engines.
Injection rates may be increased by increasing fuel injection pressures or by increasing the size or quantity of the injector nozzle openings. It is difficult to increase fuel injection pressure because diesel fuel is introduced at very high pressure. Diesel fuel injection pressures can be as high as 30,000 psi and are generally limited by injector and pump technology. Even a 2000 to 3000 psi increase in pressure would be insufficient to significantly impact injection rates.
Higher injection rates can also be achieved by increasing the injector opening size. However, increased injector opening size tends to reduce atomization of the diesel fuel, which can result in the formation of more particulates than would otherwise be the case. Increasing the number of injector openings can also lead to increases in the formation of particulates as neighboring fuel jets may interfere with one another.
While there are aftertreatment strategies for reducing the concentration of particulates in exhaust gases before those gases are expelled into the environment, particulate aftertreatment is particularly difficult and expensive to implement.
Aside from the overall emissions trade-off of utilizing EGR in diesel fuelled engines, the increased levels of particulates which result from higher levels of EGR can damage or interfere with the proper operation of components in EGR systems.
Some compression-ignition engines burn gaseous fuels such as natural gas. While such engines have a reduced tendency to generate particulates, there are other obstacles to the use of EGR in such engines. As natural gas auto-ignites at a temperature well above that needed for diesel fuel, a pilot fuel is often used to initiate combustion. Once the natural gas is ignited at a point within the combustion chamber, these natural gas fuelled engines rely on propagation of a flame front traveling from the ignition source throughout the combustion chamber to burn the fuel/air mixture.
High EGR levels can cause inefficient combustion or misfires. Maintaining a high flame speed is important for efficiency reasons. As charge-to-fuel ratio is increased, flame speed tends to fall resulting in loss in efficiency. In the limiting case, the flame speed falls to zero before the fuel is fully burned and a partial misfire occurs.
There is a need to provide reduced emission internal combustion engines.